Thick film hybrid circuit board device

ABSTRACT

A thick film hybrid circuit board device is formed by laminating a first layer and a second layer on an insulative base board. The second layer being overlapped between over the insulative base board and the first layer has a recessed portion located at cross portion where peripheral edge portion of the first layer and a peripheral edge portion of the second layer are crossed. The recessed portion is formed in the shape of being recessed inwardly of the second layer. Bleedings generated from the second layer consisting of paste are held in the recessed portion, so that short circuit accidents caused by the bleedings mutually combined are effectively prevented.

This is a continuation-in-part/divisional, of application Ser. No.07/764,062, filed on Sep. 24, 1991, now U.S. Pat. No. 5,256,836, whichis a division, of application Ser. No. 07/522,146 filed on May 11, 1990,now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a thick film hybrid circuit board deviceformed by utilizing screen printing process and to a method ofmanufacturing the same. More particularly it relates to a thick filmhybrid circuit board device having a high quality and a compactstructure and also relates to a method of manufacturing thick filmhybrid circuit boards of this character.

A thick film hybrid circuit board device of the conventional type has aconstruction which is a plurality of circuit forming materials laminatedon a insulative base board. For example, a thick film hybrid circuitboard device having a resistor element as a circuit element ismanufactured by printing, e.g., screen printing a first layer as aresistor on the insulative base board and by laminating a second layeron both peripheral edge portions of the first layer as a pair ofelectrodes.

Resistors and electrodes are usually formed by printing circuit formingmaterials on a surface of the insulative base board by utilizing ascreen printing process to form thick films.

The thick films printed on the base board are then fired underpredetermined condition to form hardened resistors and electrodes.

In the screen printing process, a screen plate having numerous microholes on the entire surface thereof if prepared as a printing mold. Theholes are provided in an outside area of a predetermined circuit patternformed by photomechanical processing are broken so as to be plugged.

The screen printing process is utilized as a method of forming a circuitfilm by squeezing a paste-like circuit forming material onto the baseboard through the holes provided inside the area of the circuit pattern.

The method of manufacturing the thick film hybrid circuit board devicewill be explained in more detail below. First,a resistive paste isscreen printed on the base board to form a layer. The printed resistivepaste is then fired to form a resistive layer as this first layer.Secondly, a conductive paste for forming a second layer is printed onboth the upper end surfaces of the resistive layer, and the printedconductive paste is fired to form a pair of electrode layers as thesecond layer.

However, in this type of the conventional thick film hybrid circuitboard device, stagger portions exist between the upper surface of theinsulative base board and a cross portion where a peripheral edgeportion of the resistive layer and that of a pair of electrode layersare mutually cross.

Due to existence of the stagger portions, a part of each of theelectrode layers formed of conductive paste will spread from an internaledge portion thereof toward the other electrode layer, resulting in theformation of bleedings which move toward the other electrode layer.

For this reason, in a case where a distance between a pair of electrodelayers narrows as the amount of integration of the thick film hybridcircuit board device increases the distance between the two electrodelayers is substantially reduced by the bleedings which comprise thepaste, so that a serious problem is encountered, that is, the resistancevalue of the resistive layer includes significant errors.

Moreover, in a case where the amount of paste bleedings generated fromthe electrode layers is large, the bleedings may mutually combine toform a bridge portion, causing a short circuit to occur between the pairof electrode layers.

Furthermore, during a trimming process for adjusting a resistance of theresistive layer, it becomes more difficult to set a starting point atwhich the trimming operation starts because the distance between thepair of electrode layers is extremely reduced.

SUMMARY OF THE INVENTION

An object of the present invention is to substantially eliminate thedefects or drawbacks encountered in the prior art described above and toprovide a thick film hybrid circuit board device which has a simpleconstruction effectively preventing variations in the electricalcharacteristics of the first layer caused by bleedings generated from apaste of the second layer and which is substantially free from shortcircuits may take place between the pair of second layers which arelocated opposite each other and, also to provide a method ofmanufacturing this type of circuit board device.

This and other objects can be achieved according to this invention inone aspect by providing a thick film hybrid circuit board device whichcomprises an insulative base board as a base plate; a first layer formedon the insulative base board; and a second layer formed so as to beoverlapped with a surface of the first layer and a surface of theinsulative base board, the second layer is provided with an inwardlyrecessed portion formed at a cross portion where a peripheral edgeportion of the first layer and a peripheral edge portion of the secondlayer are crossed.

In another aspect of this invention, there is provided a method ofmanufacturing a thick film hybrid circuit board device, the methodcomprises the steps of: preparing an insulative base board as a baseplate and at least two kinds of paste-like circuit forming materials;screen printing one paste-like circuit forming material on a surface ofthe insulative base board by utilizing a screen printing process to formone layer; firing the thus formed one layer to form as a first layer;screen printing another paste-like circuit forming material on surfacesof the first layer and the insulative base board by utilizing the screenprinting process so as to be overlapped with the first layer and theinsulative base board to form another layer having a portion recessedinwardly of the another layer having a portion recessed inwardly of theanother layer in which the recessed portion is located at a crossportion where a peripheral edge portion of the first layer and aperipheral edge portion of the another layer are crossed; and firing thethus formed another layer to form as a second layer.

According to the construction of the present invention described above,when a second layer consisting of paste is screen printed by utilizing ascreen printing process on an upper surface of a first layer formed on ainsulative base board, a recessed portion which is recessed inwardly ofthe second layer is formed at a cross portion where a peripheral edgeportion of the first layer and a peripheral edge portion of the secondlayer cross.

However, as the recessed portion of the second layer is almostcompletely filled up with bleedings generated from paste of the secondlayer, after the second layer is fired, the peripheral edge portion ofthe second layer is formed into almost original shape having no recessedportions.

Therefore, according to the present invention, variations in electricalcharacteristics of the first layer caused by bleedings generated fromthe second layer formed of paste and electrical short circuit caused bythe bleedings mutually bonded is effectively prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a partial plan view showing an embodiment of thick film hybridcircuit board device according to this invention;

FIG. 2 is a cross-sectional view taken along the line II--II of FIG. 1;

FIGS. 3 and 4 are partial plan views showing two other embodiments ofthis invention, respectively;

FIG. 5 is a partial perspective view showing a conventional thick filmhybrid circuit board device;

FIG. 6 is a cross-sectional view taken along the line VI--VI of FIG. 5;and

FIG. 7 is a partial plan view showing a condition where paste bleedingsare generated in the conventional thick film hybrid circuit board deviceshown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To better understand the preferred embodiments according to the presentinvention, the conventional technology will be described with referenceto FIGS. 5 to 7.

A conventional thick film hybrid circuit board device is manufactured inthe following manner.

Referring to FIGS. 5 and 6, first, a resistive paste containing, forexample, such as ruthenium oxide (RuO₂) is screen printed in the shapeof a rectangle on an upper surface of an insulative base board 1 made ofceramic such as alumina (Al₂ O₃) by utilizing a screen printing processto form a layer. The layer consisting of the resistive paste is thenfired at temperature of 800 to 900° C. or more preferably at about 850°C. under atmospheric pressure to form a resistive layer 2 as a firstlayer.

Next, a conductive paste containing conductive material such as copper(Cu), silver (Ag) or gold (Au) is screen printed in the shape of arectangle on the upper surfaces of both right and left edges of theresistive layer 2 by utilizing a screen printing process. The conductivepaste is then fired at a temperature of 500 to 700° C. or morepreferably at about 600° C. to form a pair of bilateral electrode layers3a and 3b as a second layer.

However, in the conventional thick film hybrid circuit board device ofthe character described in conjunction with FIG. 5 stagger portions 6aand 6b exist between an upper surface of the insulative base board 1 andeach of cross portions 7a and 7b where each of peripheral edge portions8a and 8b of the resistive layer 2 and each of the peripheral edgeportions 9a and 9b of the electrode layers 3a and 3b mutually cross.

Due to the existence of these stagger portions 6a and 6b a part of oneof the electrode layers 3a and 3b formed of the conductive paste willspread from the internal edge portion thereof toward the other electrodelayer. Consequently, as shown in FIG. 7, bleedings 4 and 4 are caused indirections towards the mutually opposing end edge portions of therespective electrode layers 3a and 3b.

For this reason, in a case where a distance between a pair of bilateralelectrode layers 3a and 3b becomes narrower, for example on the order of0.254 mm as the thick film hybrid circuit board device is moreminiaturized, the distance between the bilateral electrode layers 3a and3b is substantially reduced by the bleedings, thus providing a seriousproblem. That is the resistance value of the resistive layer 2 may havea significant error.

Moreover, in the case that the paste bleedings 4 and 4 generated fromthe electrode layers 3a and 3b are large as shown in FIG. 7, thebleedings 4 and 4 may combine with each other to form a bridge portion 5causing a short circuit between the pair of electrode layers 3a and 3b.

Furthermore, during the trimming process for adjusting a resistance ofthe resistive layer 2, it becomes more difficult to set a starting pointat which trimming operation is started because the distance between thepair of electrode layers 3a and 3b is reduced.

The present invention solves the problem of the prior art described.

The inventor has eagerly studied and found that if an amount ofbleedings generated from a paste of electrode layers is previouslyestimated and recessed portions having volumes which are equivalent tothe amount of bleedings, respectively, are formed at the each ofelectrode layers, the bleedings are received into the recessed portionsand consequently, the recessed portions are filled up with thesebleedings. The electrode layers having no outstanding bleedings arehence formed properly into the desired shape. The present invention isessentially based on these findings.

EMBODIMENTS

The embodiments in accordance with the present invention will bedescribed below in more detail with reference to FIGS. 1 to 4.

FIG. 1 shows a partial plane view of one embodiment of a thick filmhybrid circuit board device according to this invention. In FIG. 1, aresistive layer 12 as a first layer is formed on an insulative baseboard 11 formed of ceramics in the shape of rectangle. At the bilateraledge portions of the resistive layer 12 as shown in FIG. 2, a pair ofbilateral electrode layers 13a and 13b are formed on the resistive layer12 as a second layer of which a peripheral portion is overlapped to theinsulative base board 11. A resistance value of the resistant layer 12can be set to a desired value by controlling a distance (d) between thebilateral electrode layers 13a and 13b.

On the other hand, a recessed portion 14 having a rectangular shape andwhich is recessed inwardly of each of the electrode layers 13a and 13bis formed at each of four cross portions where peripheral edge portions(corresponding to both upper and lower side portions in FIG. 1) of theresistive layer 12 and peripheral edge portions (corresponding to bothend surfaces located oppositely in FIG. 1) of the pair of electrodelayers 13a and 13b are crossed.

Due to existence of stagger portions between each of the electrodelayers 13a and 13b and the insulative base board 11, the bleedings inthe areas marked as 4 are generated from both electrode layer consistingof paste as shown in FIG. 7 spread from one electrode layer toward theother electrode layer mutually located int he opposing condition.Therefore, an amount volume of the bleedings 4 is previously estimatedand each of the recessed portions 14 having a volume which is equivalentto the amount of each of the bleedings 4 are formed at each of theelectrode layers 13a and 13b.

Namely, the volume of the recessed portion 14 is set to the value whichis equivalent to the amount of bleeding 4 generated from the electrodelayers 13a and 13b as a second layer printed on the resistive layer 12as a first layer.

Accordingly, when the pair of electrode layers 13a and 13b are screenprinted on the surfaces of the resistive layer 12 and the insulativebase board 11 by utilizing, for example, the screen printing process,the conductive paste of each of the electrode layers 13a and 13b willspread in each of the recessed portion 14 formed in the electrode layers13a and 13b to form bleedings.

However, almost part of the bleedings is received into the recessedportions 14. Consequently, the recessed portion 14 are almost filled upwith the bleedings. Accordingly, the electrode layers 13a and 13b eachhaving a plane and parallel edge portions are obtained, and theelectrode layers 13a and 13b are finally formed so as to be close to theoriginal shape having no recessed portions showing in FIG. 5.

For example, in the case of that the distance (d) is about 0.254 mm(0.01 inch), a cutout depth (h) of each of the recessed portions 14 and14 may be set to about 0.05 mm respectively, and the total amount of thecutout depth is set to about 0.1 mm so that each of the recessedportions 14 will be filled up with the bleedings generated from theconductive paste of each of the electrode layers 13a and 13b. As theresult, both opposite edge surfaces of the electrode layers 13a and 13bbecome almost parallel with each other, and the distance (d) can beexactly set to a desired value of about 0.254 mm, whereby apredetermined resistance value is secured.

Next a further embodiment of the method of manufacturing the thick filmhybrid circuit board device constructed in the manner described abovewill be described.

First, an insulative base board made of ceramic such as alumina (Al₂ O₃)is prepared as a base plate, and at least two kinds of circuit formingmaterials for forming circuit elements such as resistor, electrode orwirings are prepared in the state of paste which is made enable to flowthrough the micro holes of the screen during screen printing process.

In this embodiment, a resistive paste containing, for example, rutheniumoxide (RuO₂) powder and a conductive paste containing cooper (Cu) powderare prepared as circuit forming materials for forming resistors andelectrodes respectively. Secondly, the resistive paste is screen printedby utilizing the screen printing process through the screen plate on theinsulative base board 11 to form a printed resistive layer in variousshapes such as a rectangle. Thirdly, the printed resistive layer isfired at a temperature of 800 to 900° C. or more preferably at about850° C. under normal atmospheric pressures to form a resistive layer 12as a first layer.

Fourthly, the conductive paste is screen printed on both right and left(in FIG. 1) edge portions of the resistive layer 12 so as to form a pairof bilateral electrode layers 13a and 13b as a another layer byutilizing the screen printing process.

At the screen printing operation, the recessed portion 14 is formedthrough a screen plate at each of four cross portions respectively whereperipheral edge portion of the resistive layer 12 (corresponding to bothupper and lower side portions in FIG. 1) and peripheral edge portions ofthe electrode layers 13a and 13b are mutually crossed orthogonally. Thescreen plate for making a circuit pattern in previously constructed soas to form the recessed portions 14 and 14.

However, after the screen printing operation, each of the recessedportions 14 is substantially filed up with the bleedings generated froma pair of electrode layers 13a and 13b consisting of conductive paste,whereby the recessed portions 14 are formed almost into a flat shape.

Then, the printed electrode layers 13a and 13b formed of conductivepaste are fired at the temperature of 500 to 700° C. more preferably atabout 600° C. in N₂ gas atmosphere, for example, to form a pair ofbilateral electrode layers 13a and 13b, respectively.

According to this embodiment, the distance between the pair of bilateralelectrode layers 13a and 13b does not become narrower and is set to apredetermined value, so that the resistance value of the resistive layer12 can be set precisely to a desired value.

In addition, electrical short circuit which would take place in a pairof electrode layers 13a and 13b caused by the formation of a bridgeportion 5 as shown in FIG. 7, which is formed by the bleedings 4mutually combining are effectively prevented.

In the embodiment described above in the case where the recessed portion14 of the pair of bilateral electrode layers 13a and 13b are formed ascutout portions in the shape of rectangle. However, the presentinvention is not limited to the described embodiment. For example, asshown in FIG. 3, recessed portions 14a may be formed by cutting off theelectrode layers 13a and 13b so as to have a shape of a triangle. Inthis case, as only one straight cutting line extends diagonally isdrawn, it therefore becomes easy to design a pattern for making therecessed portion.

In addition, as shown in FIG. 4, recessed portions 14b may also beformed by cutting off the electrode layers 13a and 13b in the shape of asemicircle. In this case, each of the electrode layers 13a and 13b isset respectively so as to match an edge portion of the resistive layer12 with a center line of the semicircle. In other words, each of theelectrode layers 13a and 13b is set so as to maximize a length (W) ofthe edge portion being cut off by the semicircle.

Accordingly, it becomes easy to set a initial locating point of theelectrode layers 13a and 13b against the resistive layer 12.

Moreover, in the embodiment described above, the method of manufacturingof circuit board device having a resistive layer 12 as a first layer andhaving a electrode layers 13a and 13b as a second layer is introduced.However, the present invention is not limited to this embodiment.

The present invention can be generally adapted in the same manner to allkind of this thick film hybrid circuit board device formed by laminatingmore than three circuit element material layers.

It is to be understood that the present invention is not limited to thepreferred embodiment described above and many other modifications andchanges may be made without departing from the scope of the appendedclaims.

What is claimed is:
 1. A thick film hybrid circuit board devicemanufactured by utilizing a screen printing process, said circuit boarddevice comprising:an insulative base board as a base plate; a firstlayer formed on said insulative base board; a second layer formed so asto be overlapped with a surface of said first layer and a surface ofsaid insulative base board, said second layer being provided with aninwardly recessed portion formed at a cross portion where a peripheraledge portion of said first layer and a peripheral edge portion of saidsecond layer are crossed; and a bleeded portion at least partiallyfilling said recessed portion said bleeded portion being generated fromsaid second layer.
 2. A thick film hybrid circuit board device accordingto claim 1, the recessed portion is substantially filled up with saidbleeded portion.
 3. A thick film hybrid circuit board device accordingto claim 1, wherein said recessed portion is formed by cutting off insubstantially a triangular shape said second layer.
 4. A thick filmhybrid circuit board device according to claim 1, wherein said recessedportion is formed by cutting off in substantially a semi-circular shapesaid second layer.